Pyrochlore-type ternary oxides tl2ru207 and tl2ir207

ABSTRACT

DESCRIBED AND CLAIMED ARE THE TWO TERNARY COMPOUNDS TL2RU2O7 AND TL2IR2O7, BOTH OF WHICH HAVE THE PYROCHLORETYPE STRUCTURE. THE COMPOUNDS ARE PRODUCED FROM MIXTURES OF THE BINARY OXIDES, TL2O3 AND RUO2 OR IRO2, RESPECTIVELY, IN SUBSTANTIALLY THE STOICHIOMETRIC RATIOS AT TEMPERATURES OF 400*C. TO 1,400*C. THE COMPOUNDS ARE USEFUL IN ELECTRICAL APPLICATIONS.

United States Patent 01 3,560,144 Patented Feb. 2, 1971 ice 3,560,144 PYROCHLORE-TYPE TERNARY OXIDES Tl Ru O Tlgllgoq Arthur W. Sleight, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed June 28, 1968, Ser. No. 741,220 Int. Cl. C01b 15/00 U.S. Cl. 23-50 3 Claims ABSTRACT OF THE DISCLOSURE Described and claimed are the two ternary compounds Tl Ru O and Tl Ir O both of which have the pyrochloretype structure. The compounds are produced from mixtures of the binary oxides, T1 and RuO or IrO respectively, in substantially the stoichiometric ratios at temperatures of 400 C. to 1,400 C. The compounds are useful in electrical applications.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to, and has as its principal object provision of, the two compounds TlgRUgOq and Tl Ir O useful in electrical resistors.

(2) Description of the prior art Over one hundred mixed oxides are known to have the pyrochlore structure. Consequently, this summary is not exhaustive insofar as compounds with this structure are concerned. Those most closely related to Tl Ru O and Tl Ir O are, however, believed included. Ternary oxides of the type R M O with the pyrochlore structure are known where R is a rare earth and M is Ti, Zr, Hf, Tc, Re, Ir, Ge, and Sn as well as where R is Cd or Hg and M is Nb, Re, Ta or Sb. The defect pyrochlores, AgS'bO PbRuO PbTcO PbIrO and BiRhO are also known. Recently, the pyrochlore phase Tl Pt O has been reported by Hoekstra and. Siegel, Inorganic Chemistry 7, 141 (1968). Bi Ru O has the pyrochlore structure and has been found to be useful in resistor compositions: see the copending, coassigned, U.S. Pat. application of Hoffman, Ser. No. 869,351, filed Oct. 24, 1969, as a continuation-in-part of Ser. No. 748,952, filed July 31, 1968, as a continuation-in-part of Ser. No. 701,016, filed Jan. 26, 1968, the latter two applications now abandoned. Compositions containing Tl O and RuO are also known to be useful as resistors: see U.S. Pat. 3,352,797.

DESCRIPTION OF THE INVENTION The novel compounds of this invention are two in number, having the formula Tl X O where X is Ru or Ir. Both compounds have the pyrochlore-type structure.

The two new compounds are prepared from T1 0 and RuO or IrO respectively. The latter two oxides may be in either crystalline or amorphous form, the thallium oxide apparently being known only in crystalline form. In practice, the compounds are prepared by mixing the respective reactants and grinding them together with a mortar and pestle or similar device. The reaction mixtures are then simply heated together in an acceptable inert container. When a closed system is employed, the stoichiometric molar ratios are 1:2 of Tl O :other oxide. When open systems are used, however, a slight excess of T1 0 i.e., a mole ratio of around 1.05:2, can be employed since T1 0 becomes significantly volatile at around 600 C. and rapidly volatile around 700 C.

The lower temperature limit for preparing these compounds is determined by the reactivity of the reactants and is in the neighborhood of around. 400 C. The upper temperature limit is around 700 C. in an open system but may be extended to well over 1,000 C. in a closed system. The preferred temperature range, however, is about 500700 C.

A closed system is useful since it prevents the loss of T1 0 by volatilization. It is sufiicient that the pressure equal the vapor pressure of T1 0 At 700 C., for example, 1 atmosphere or autogenous pressure is sufiicient. Higher pressures, particularly at higher temperatures, can, of course, be employed.

In an open system, the atmosphere is largely immaterial and can be oxygen, air, or any inert gas. Oxygen, however, is preferred since it will oxidize any trace of iridium or ruthenium present.

The minimum time required to prepare Tl Ru O and Tl Ir O depends on the reactivity of the reactants, the degree of mixing, the temperature of the reaction, and, to a lesser extent, the reaction pressure. Thus, at 1,000 C. in a closed system, five minutes is sufi'icient while at 400 C. ten hours may be required to reach the same degree of reaction. In open systems there will be slow volatilization (mainly of T1 0 so that a practical upper limit is about 24 hours.

The reaction preparing the compounds of the invention is preferably carried out in a gold reactor. Other inert metals, such as precious metals, and ceramics can be used if desired, however.

EMBODIMENT OF THE INVENTION There follow some nonlimiting examples illustrating the preparation of the compounds of the invention in more detail. In the examples, percentages are by weight.

Example 1 Preparation of Tl Ru O Thallium sesquioxide,

" T1 0 and ruthenium dioxide RuO were weighed out in amounts of 0.6525 g. and 0.3802 g., respectively and the two oxides were mixed by grinding them together in an agate mortar and pestle. The mixture was placed in a gold tube (5" long, diameter, .010" wall) which had been sealed at one end by welding. The other end of this tube Was then also welded shut.

The sealed gold tube was placed in a pressure vessel. The pressure was increased to 3,000 atmospheres, and the temperature was increased to 700 C. These conditions were held for eight hours. The pressure vessel and tubes were cooled quickly to room temperature and the pressure was released.

On opening the gold tube, black, polycrystalline Tl Ru O was recovered. An X-ray diffraction pattern of this powder could be completely interpreted on the basis of a face centered cubic unit cell with an edge of 10196:.001 A. The data further indicated that the crystal structure was undoubtedly of the pyrochlore type. Since no impurities were detected, purification was unnecessary.

Example 2 Preparation of Tl Ir O .Tallium sesquioxide and iridium dioxide, IrO were weighed out in amounts of 0.5075 g. and 0.4982 g., respectively, and a preparative procedure otherwise identical in all respects with that of Example 1 was carried out.

Black, polycrystalline Tl Ir O was recovered from the gold tube. An X-ray powder diffraction pattern of the product showed a face-centered cubic unit cell with an edge of 10.21:.01A. Although some impurities were noted, it was apparent that a phase with the pyrochlore structure was present in better than yield.

Example 3 Preparation of Tl Ir O under oxygen-Thallium sesquioxide and iridium dioxide were weighed out in Tests using the two compounds of this invention in gold-containing resistors of the type described in application Ser. No. 701,016 are given in the following table. The compositions set forth in the table were made as follows:

The pyrochlore, glass, and gold phases were mixed together by grinding and then into a paste by mixing with 30% by volume of fi-terpineol. The paste was screen-printed on a ceramic body, and the system was heated slowly to 200 C. and then fired at about 750 C. for about ten minutes.

TABLE TCR TCR +25 Resistance per to +125 0., to 75 0., Composition of resistor square p.p.m. p.p.m.

45% Tlgllg07, 10% An, 45% glass b 1.26 meg-ohm. 260 -382 Tl2lrzO 20% Au, glass .944 meg-ohm. -15 45% T12R112O7, 10% An, 45% glass 1.30 k-ohm +522 +410 35% Tl2Ruz01, 20% An, 45% glass 805 ohm +810 +480 a TC R Thermal eoefiicient of resistance. b The glass used is 65% PbO, 26% S102 and 10% B20 0 p.p.m.=PartS per million.

Analysis.Calcd. for Tlglrgoq (percent): Tl, 45.16; Ir, 42.47; 0, 12.4. Found (percent): Tl, 48.04; Ir, 42:2; 0, 12.7.

Example 4 Preparation of Tl Ru O under oxygen.Thallium sesquioxide and ruthenium dioxide were weighed out in amounts of 2.3979 g. and 1.3307 g., respectively, and a preparative procedure otherwise identical to that of Example 3 was carried out. The black polycrystalline Tl Ru O recovered gave an X-ray powder diffraction pattern essentially identical to that obtained in Example 1.

Analysis.-Calcd. for Tlzlrzoq (percent): Tl, 45.16; Ru, 27.96; 0, 15.5. Found (percent): Tl, 56.11; Ru, 28.7, 29.0; 0, 15.9.

Both Tl Ru O and Tl Ir O are useful in electrical resistor compositions of the type shown, for example, by DAndrea, U.S. Pat. 2,924,540, and Dumesnil, U.S. Pat. 3,052,573. See also the above-mentioned Hoffman application Ser. No. 701,016. When used in such compositions, the present novel compounds have an advantage in that they have very low thermal coefficients of resistance (TCR), i.e., the electrical resistivity of the compositions changes 'very little as a function of temperature.

Resistors made with TlzRllzoq and Tl Ir O are also particularly resistant to changes in humidity. Resistance to humidity can be tested by exposing the resistor composition to 95% relative humidity at C. overnight, and determining the change in resistivity.

Because of its high resistivity, Tl Ir O is the preferred product of the two compounds claimed.

Since obvious modifications and equivalents in the invention will be evident to those skilled in the art, I propose to be bound solely by the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A compound of the formula T12X2O7 wherein X is Ru or Ir, said compound having the pyrochlore 30 structure.

2. The compound of claim 1 wherein X is Ru,

3. The compound of claim 1 wherein X is Ir,

References Cited High Pressure, Inorganic Chemistry, January-June (1968), pp. 141-145; paper presented at 153rd National Meeting of American Chemical Society, Miami Beach, Fla., April 1967.

' OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner U.S. Cl. X.R. 2315 

